Shape measurement of a three-dimensional object using an optical system is becoming more popular in various industrial fields. Of these methods, a light-sectioning method is known and is considered to be a highly practical method. In a light-sectioning method, reflection light from a target surface when slit-shaped or spot-shaped light is irradiated on the target is captured by a camera, and a three-dimensional coordinate of each point on the surface of the target is determined from a positional relationship between a light source and the camera, through triangulation.
As a device for measuring three-dimensional shape through a light-sectioning method, there is known a device, for example, described in Non-Patent Document 1 (in particular, refer to “3.2.3 Image Encoder”). In this device, an image formed when slit light is swept over the target is captured by a CCD camera, and a video signal output by the camera is input to an image encoder. In addition to the video signal, a coding signal indicating a projection angle of the slit light is input to the image encoder. The image encoder applies a peak-holding process of brightness for each pixel in real time with respect to the input video signal, and, at the same time, detects a timing when each pixel is at the maximum brightness as determined by the peak-holding process, stores the coding signal at the detected timing as a coding value of the pixel, and forms a coding image to which the light-sectioning method can be applied.
The device of Non-Patent Document 1, however, assumes that the projection position of the slit light remains substantially unchanged during one reading and scanning period of the CCD camera. For the shape measurement of an object, the reading and scanning process must be repeated while the position of the slit light is changed gradually. Because a reading and scanning period of a typical CCD camera is 1/30 seconds or 1/60 seconds, three-dimensional shape measurement of a target by this device requires a long period of time, and, thus, the device substantially cannot be applied to a moving object.
In consideration of such a disadvantage, one of the present inventors proposed a device described in Patent Document 1. This device comprises a non-scanning image-capturing element having an element memory for each light detector. Each element memory is connected to a bus for transferring a time elapse signal t. A light reception output which is output when the light detector detects the slit light is supplied to the element memory as a trigger, and the element memory latches a value of the time elapse signal t supplied from the bus at that point of time. With such a configuration, when the slit light is swept once with respect to the target, information corresponding to time during which the slit light is incident on the corresponding light detector is stored in each element memory of the image-capturing element. In other words, in this device, information similar to the coding image generated by the image encoder of Non-Patent Document 1 is stored with respect to the element memories of the image-capturing element during one sweep of the slit light. Therefore, it is sufficient to perform a read and scan process of the memories of the image-capturing element once for each sweep of the slit light. In this manner, according to the device of Patent Document 1, the shape can be measured even when the slit light is swept at a speed similar to that of the read and scan process of the image-capturing element, and, thus, the shape can be measured almost in real time.
In the device of Patent Document 1, however, because an element memory must be provided for each light detector of the image-capturing element, there are disadvantages such as, for example, the size of the image-capturing element is increased and manufacture becomes complicated.
Non-Patent Document 2 describes a related art technique which takes another approach for handling the above-described disadvantage. In this reference, the real-time capability is enhanced through high-speed reading of the image-capturing element. A CMOS sensor that is used has, in addition to a light detector array, an analog memory array which can store signals of 4 frames for each pixel (a pixel in a 4-color light detector), and a comparator and an output latch provided for each column of the memory array. In this sensor, a signal of each pixel is read at a high frame rate of 3.3 kfps (kilo-frames per second), and the read signal is stored in a cell of a corresponding frame of the analog memory array. For the pixels of each column of the array, a difference signal is sequentially determined by subtracting, using the comparator for the column, a sum of two frames which are earlier in time from a sum of two frames which are later in time among the signals of four frames stored in the analog memory array, and a value of “0” is stored in the output latch when the difference signal is 0 and a value of “1” is stored in the output latch when the difference signal changes from a negative value to 0 (this is the timing when the peak of the slit light is on the pixel). By reading the output latch of each column at a high speed, it is possible to determine whether or not the peak of the slit light is incident on the pixel for each frame (one read and scan period of the light detector array). A frame number corresponds to a projection angle of the slit light, and, because it is possible to determine, by means of the sensor, the frame of each pixel at which the slit light peak is detected, the three-dimensional shape of the target can be determined.
The sensor of Non-Patent Document 2 can achieve an improvement in the aperture ratio of the light detector array by providing the circuit structures such as the analog memory array, the comparator, and the output latch outside of the light detector array. However, because an analog signal memory of 4 frames must be provided for each pixel, there is a disadvantage that the circuit of the element is increased in scale. Even if 4 frames per pixel is only exemplary, because a principle of determining a peak of slit light by a temporal difference in the light reception signal of the same pixel is employed, an analog signal memory of at least 2 frames must be provided for each pixel.
Patent Document 2 discloses, as an image-capturing element for three-dimensional measurement, a structure in which a plurality of pixels are arranged two-dimensionally, each pixel has an elongated shape along the sweep direction on the image-capturing surface of the slit light, and pixels adjacent along a direction perpendicular to the sweep direction are shifted from each other along the sweep direction. This reference also discloses a method in which output values of adjacent pixels are compared, a spatial position on the image-capturing element with the difference inverted is stored, and the pixel on which the slit light is incident is identified (refer to paragraphs 7 and 26).
However, Patent Document 2 fails to disclose a specific hardware circuit structure for realizing the method.
Patent Document 3 discloses, as a device for determining a three-dimensional shape of a target, a device in which a zeroth order moment and a first order moment at each position along a direction of extension of pattern light in a direction perpendicular to the direction of extension of the light are determined by means of a zeroth order moment calculator and a first order moment calculator, by integrating digital signals corresponding to output signals obtained at approximately the same time in the light-receiving element. In this device, an opto-electric output of each light-receiving element of the light detector array is converted to a multi-value digital signal, and a moment calculation process is applied to the digital signal.
The device of Patent Document 3 requires a complex circuit structure for various processes such as analog-to-digital conversion of the opto-electric output of each light-receiving element and parallel reading of obtained digital values, and, thus, the manufacturing cost of the device of Patent Document 3 is high.    Non-Patent Document 1: Toru Yoshizawa, “Three-Dimensional Engineering 1—Optical Three-Dimensional Measurement”, 1st Ed., Shingijutsu Communications Inc., March, 1993, p. 38-51.    Non-Patent Document 2: Toshinobu Sugiyama, and three others, “A Color Imaging and Real-time 3-D Sensing CMOS sensor”, ITE Technical Report, The Institute of Image Information and Television Engineers, Mar. 18, 2002, Vol. 26, No. 26, pp. 1-6.    Patent Document 1: JP 6-025653 B    Patent Document 2: JP 2001-053261 A    Patent Document 3: JP 2002-365022 A